The quantitative study of how fluids (gases and liquids) move.

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Determine the pressure difference required to drive a prescribed constant volume flux $Q$ through a gap

Determine the pressure difference $P_{1}-P_{2}$ required to drive a prescribed constant volume flux $Q$ per unit width through a gap of thickness $\delta$ of length $L$. To do this introduce ...
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Calculating Reynolds number for a viscous droplet

I'm trying to develop a very basic scaling law/unit analysis for viscous droplet formation, and I'd like to get some rough numerical values of the Reynolds number to play with. To be specific, I'm ...
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4answers
27k views

What causes ballpoint pens to write intermittently?

After a while, a ball point pen doesn't write very well anymore. It will write for a little distance, then leave a gap, then maybe write in little streaks, then maybe write properly again. It seems ...
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88 views

What is this form of mass conservation equation?

I found the following equation of conservation of mass (continuity) in "Computational Fluid Dynamics Vol.III" by Hoffmann: $$ \frac{\partial \rho}{\partial t} + \frac{\partial}{\partial x}(\rho u)+ \...
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163 views

Simple cylinder fluid mechanics, draining from hole on bottom

So my professor is having us work on an assignment where there is a cylinder full of water with a hole in the side near the bottom. Given the use of stopwatch, balance beam, beakers, etc, how can we ...
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332 views

Clarification on slip, no-slip and no-penetration for incompressible flow

This is my current understanding: The no-slip condition at a boundary means that there is no velocity relative to the boundary, this means that the individual components is zero. So we have (in two ...
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135 views

How are the turbulent spectra determined in relativistic turbulence?

In a non-relativistic compressible fluid, the turbulent energy spectra are well-understood and appear to follow the Kolmogorov hypothesis. It would also appear that relativistic turbulence also ...
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29 views

Are these reasonings about tides correct?

This is a picture of the "M2 tidal constituent": The nodes corresponds to fixed points, for which the moon has no influence on the altitude. 1. Static VS Dynamic problem? I was first wondering if ...
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63 views

Continuum fluid theory

It is written in this article: http://www.maths.ed.ac.uk/~yktsang/4520/basic_fluid.pdf "In the continuum model of fluids, physical quantities are considered to be varying continuously in space, for ...
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161 views

How do shock waves form in space?

Today my aerodynamics professor mentioned that the equations we were learning for atmospheric shock waves can also be applied by astrophysicists to study black hole-related shock waves in space. How ...
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85 views

How to model the mixing of two fluids?

In the context of a research project of mine, I am faced with the difficult task of modeling the mixing of two fluids in a container. I would like to achieve the following: Given a container (...
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72 views

Fluid parcel + point of a fluid

If we have a fluid(Static or dynamic), and for example we pick a point in this fluid. This point is represented by a very small cube (or small volume). Inside this cube, it has the same properties (...
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305 views

Why the units of surface tension are force per unit length?

I do not understand surface tension concept. I read explanations from many sources but I still do not get the units that it has. The units of surface tension are $N/m = J/m^2$. Let's define the ...
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479 views

Why do hot tubs dehydrate you? [closed]

I was just sitting in a hot tub for a few hours with friends, and we all noticed we really wanted to drink some water. We felt dehydrated. I tried looking up why hot tubs dehydrate you and all I got ...
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8answers
6k views

Does it take significantly more fuel to fly a heavier airplane?

I was reading in the papers how some-airline-or-the-other increased their prices for extra luggage, citing increased fuel costs. Now I'm a bit skeptical. Using the (wrong) Bernoulli-effect ...
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3answers
611 views

How can the atmospheric pressure be different in distinct points at the same altitude?

From an hydrostatic point of view, the pressure in a fluid should be the same at the same depth/altitude. Obviously, in our atmosphere that does not happen. I am guessing that the main reason is the ...
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2answers
275 views

Fluid mechanics assumptions

I have just started studying fluid mechanics and on the very first page two assumptions are made about liquids while I understood the first one but am not able to understand the Second one //" parts ...
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70 views

Bernoulli principle and particle

Bernoulli principle describes the flow of a fluid for steady, incompressible flow along a streamline. But it is said for a particle of a fluid along a streamline. My question is a particle of fluid ...
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21 views

Speed increase in a ventrui relative to its initial speed

Does the air speed through a ventrui reach a higher final speed if its speed prior to entering the ventrui is higher than if it entered the same ventrui with a lower speed? I should add that the ...
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Angular momentum transfer in a bottle vortex

A colleague and I were discussing the following fluid mechanics experiment, which most people have probably tried in their kitchen: take a bottle full of water (a 2-litre transparent soda pop bottle ...
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What is the mystery of turbulence?

One of the great unsolved problems in physics is turbulence but I'm not too clear what the mystery is. Does it mean that the Navier-Stokes equations don't have any turbulent phenomena even if we solve ...
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1answer
140 views

How to calculate gas flow/permeation through solid metal membrane

Given the absolute pressures of air on both sides of a titanium membrane of known thickness (0.1mm) and diameter (3mm), how would one calculate the rate of gas exchange/escape through the titanium ...
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102 views

I disagree with the definition of Reynolds number. Why am I wrong?

My question is about the intuition behind the Reynolds number. For example they say that if a fluid is moving 1 10 mph past a sphere of radius 10m with viscosity v 2 100 mph past a sphere of ...
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78 views

What does $\mathbb{R}^3$ and $\mathbb{T}^3$ look physically for the Navier-Stokes equation?

What does the Navier-Stokes equation solution according to the Clay Math Institute look like in real life? As in how do you visualize $\mathbb{R}^3$ and $\mathbb{T}^3$ without the math? I actually ...
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222 views

Sound Propagation using Finite-Difference Time-Domain (FDTD) considering wind effects

I am trying to plot the propagation of sound from a fixed source in a 2D environment using Finite-Difference Time-Domain (FDTD) Method taking into account the effects of the wind velocity. I came ...
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5answers
156 views

Pressure variation

Imagine we have a vertically orientated pressurised liquid filled closed pipe. The pipe is shaped like a spring, such that the distance between the two ends can be adjusted vertically, whilst the ...
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Falling then rising of an effervescent tablet dropped into a glass of water [duplicate]

If you drop an effervescent tablet into a glass of water. The tablet first sinks to the base of the glass and effervesces energetically - producing a rising column of bubbles, i.e. a bubble plume - as ...
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Should cohesion be considered as a resistant force here?

In some droplet generators, a pressure pulse is applied on top of a liquid reservoir connected to a nozzle and FIRST, a liquid jet is emerged from the nozzle and THEN, pinch-off of a droplet will ...
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62 views

Why does rolling down opposite windows create a breeze?

If, while driving, I roll down both of the front windows an inch or two, it mainly creates some turbulence around the window. By symmetry we shouldn't really expect anything different. However, if I ...
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6k views

Whistle Physics

I'm looking for a simple explanation of how a whistle operates. I know that forcing air over a sharp lip can set up a wave in a resonating cavity, but how? "Most whistles operate due to a feedback ...
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1answer
75 views

Would a propeller underwater have thrust?

The type I mean is the ones on aircraft. Do the same physics apply with a propeller underwater?
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107 views

Bernoulli equation as an energy conservation equation

I don’t want to learn and apply Bernoulli's equation. It is complicated and you need to remember a lot. I instead want to apply energy conservation. But I don't always find it easy. For example, when ...
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51 views

Is there a formulation for (self-)accelerating fluid flow through permeable medium?

I have a permeable system where is an accelerating fluid flow. Imagine a sponge that is squeezed. The fluid starts at rest, accelerates and flows out from the sponge. How to calculate the fluid speed? ...
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Fluid Mechanics with calculus on manifolds

Fluid Mechanics is a branch of physics that uses a lot of vector calculus in $\mathbb{R}^3$ to describe phenomena mathematically. Calculus on manifolds, however, is the straightforward generalization ...
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Why do non-Newtonian fluids go hard when having a sudden force exerted on them?

You can dip your hands into a bowl of non-Newtonian fluid but if you are to punch it, it goes hard all of a sudden and is more like a solid than anything else. What is it about a non-Newtonian fluid ...
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96 views

If pressure in fluids is caused by the elastic collision of fluid molecules then why does the pressure increase with depth?

We know that pressure in fluids increases with depth but why does it happen, if pressure is caused by the collision of particles?
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244 views

How to calculate thrust from opening a pressure vessel in space?

One popular trope in science fiction is the astronaut who (deliberately or inadvertently) punctures an air tank or something and goes rocketing off. How would we calculate the amount of force that ...
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Non-Newtonian Fluid Stop a Bullet?

I just saw a YouTube video about Non-Newtonian fluids where people could actually walk on the surface of the fluid but if they stood still, they'd sink. Cool stuff. Now, I'm wondering: Could a pool ...
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159 views

Htting the brakes with a pickup truck full of water

I am looking to settle a bet. My friend and I were discussing truck bed pools, which are pick-up trucks with their beds tarped and filled with water converting them into small temporary swimming pools....
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1answer
51 views

Will there be a drag for a solid body already dragged due to reaction?

By learning Fluid Structure Interaction tutorials, I managged to simulate a 2d transient fsi simulation using Comsol fsi fully coupled module. A solid body due to motion in vertical direction ...
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141 views

Calculating Reynolds number of ellipse in fluid

I know Reynolds number is calculated from the formula by including length or diameter of an object in fluid. I have a case where there is ellipse with 2a major and 2b axis. I hv learnt when the ...
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100 views

How is the pressure of an incompressible fluid measured?

I've googled around and can't find quite what I'm looking for.. this may be a VERY basic question, but I'd appreciate if it could be explained in layman's terms.. Traditional fuel injection engines ...
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3answers
261 views

Angular velocity of a fluid element

If we have a fluid element that is subjected to: Translation Rotation Extensional strain (dilatation) Shear strain As in this picture starting from time $t$ it can be shown that angles $d\alpha$...
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2answers
196 views

Must-know topics for Computational Fluid Dynamics (CFD)

My undergrad major was Electrical Engineering. My MS thesis is related to simulation of hydrogen movement in a complex geometry. I did take a course on Fluid Dynamics but I have to say it wasn't one ...
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52 views

How long can water hold elevated pressure and energy after opening pressure cooker lid?

I noticed today that the water warmed up in pressure cooker (1.5 ATM) dissolves sugar much faster and much more than water cooked in 1 ATM. I am interested in how long can water hold the high ...
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166 views

What is the physical origin of acoustic modes in a duct?

Lets deal with wave propagation in a cylindrical duct. We ask the question: "what is the general form of a pressure wave which can propagate through the duct?" In answering this, we assume that the ...
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207 views

Is there a theoretical limit to the maximum flow through a pipe?

Assuming laminar flow, the Hagen–Poiseuille equation defines the flow rate as a function of the head pressure. However as the flow rate increases so does the head loss due to friction within a pipe, ...
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2answers
2k views

Pump pressure required to fill water tower

I have a water pump at a distance of x meters from a water tower located h meters above the pump through a pipe with diameter d. I am not concerned with the exact value just want to make sure my pump ...
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Is there a dynamic prelude to the state equation?

If I'm not mistaken, the state equation $$PV=nRT$$ also known as the 'ideal gas law' serves as a mathematical model relating pressure-volume-mass-temperature states of an assumed ideal gas. In other ...
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644 views

Continuity equation and pressure head in a tank

Consider a tank which is full of water. We connect a horozontal pipe at its bottom. When the pipe is open, water starts flowing through the pipe, then the velocity of water is proportional to the ...